1. Field of the Invention
The present invention relates to a display apparatus configured in such manner that multiple driver Integrated Circuits (ICs) are connected to the periphery of a display panel such as a liquid crystal display panel for the purpose of supplying signal input terminals formed on its periphery with signals.
2. Description of the Prior Art
For some time now, the Tape Carrier Package type mounting structure (TCP) has been employed as a mounting structure to connect driver ICs to a liquid crystal display panel for the purpose of supplying signals to signal input terminals formed on the periphery of the liquid crystal display panel. The TCP is constituted by disposing on a flexible circuit board the driver ICs, signal input wires for supplying the driver ICs with external signals, and signal output wires for supplying the liquid crystal display panel with drive signals from the driver ICs. The external signals include image data, an analog power supply voltage used to drive the driver ICs, and gradation power supply voltages for a gradation display.
If a liquid crystal display apparatus of the TCP type is the active matrix type using TFTs (Thin Film Transistors), gate TCPs and source TCPs are connected to the periphery of the liquid crystal display panel for supplying gate signal lines and source signal lines of the liquid crystal display panel respectively with signals, and an external circuit board is connected to the respective TCPs for the purpose of supplying external signals. The signal input wires of the TCP are connected to output terminals of the external circuit board, and the external signals are supplied from the external circuit board to the driver IC. The signal output wires of the TCP are in turn connected to the signal input terminals of the liquid crystal display panel, and the drive signals are supplied from the driver ICs to the liquid crystal display panel.
However, since the external signals are directly inputted from the external circuit board to the respective TCPs, it is necessary to provide a very large number of wires on the external circuit board of the liquid crystal display apparatus of the TCP type, which consequently involves a complex manufacturing process and increased costs while decreasing the level of reliability.
In view of the aforementioned problems, a so-called signal transmission type has recently been introduced to replace the TCP type described above, and the signal transmission type sequentially transmits external signals which have been inputted to one TCP from an external circuit board, and to an adjacent TCP. The liquid crystal display apparatus of the signal transmission type is described in Japanese Laid-Open Patent Publication No. 2001-56481.
In the liquid crystal display apparatus of the signal transmission type, the TCP is constituted by disposing on a flexible circuit board, a driver IC, signal input wires which supply the driver IC with external signals, signal output wires which supply the liquid crystal display panel with drive signals from the driver IC, and relay wires which output external signals required for driving the liquid crystal display panel to an adjacent TCP. On the other hand, the TCPs are connected to the periphery of the liquid crystal display panel, while connection wires, which electrically connect two adjacent TCPs are laid out on the gap formed between these TCPs.
In the liquid crystal display apparatus of the signal transmission type, the external signals supplied from the external circuit board are inputted to a specific TCP. In the TCP to which the external signals are inputted, the necessary external signals are supplied to the driver IC via the signal input wires, while the drive signals are supplied from the driver IC to the liquid crystal display panel via the signal output wires. Moreover, in the TCP to which the external signals are inputted, a portion of the external signals is supplied to relay wires, and such external signals are then supplied to the signal input wires of the adjacent TCP via the connection wires on the liquid crystal display panel. Similarly, in the adjacent TCP, required external signals are inputted to the driver IC, and a portion of the external signals is supplied to an adjacent TCP via the relay wires and the connection wires.
Accordingly, the external signals from the external circuit board are inputted to the specific TCP and sequentially transmitted from this specific TCP to the adjacent TCPs in the liquid crystal display apparatus of the signal transmission type.
In contrast to the conventional liquid crystal display apparatus of the TCP type, it is possible to substantially reduce the number of wires required for inputting the external signals from the external circuit board to the TCPs in the case of the liquid crystal display apparatus of the signal transmission type, resulting in a decrease in manufacturing cost.
However, since the liquid crystal display apparatus of the signal transmission type sequentially transmits the external signals which are input to the specific TCP, to the adjacent TCPs, the signal wires used for transmitting the external signals necessarily become longer, leading to increased wire resistance. Moreover, since a large number of signal wires is required in the TCPs and the liquid crystal display panel to be used for the driver ICs and for driving the liquid crystal display panel, the space required to dispose the signal wires for transmitting the external signals is therefore restricted, making it impossible to increase the width and thickness of the signal wires, thereby leading to greater wire resistance. Further, the increase in resistance of the signal wires occasions a drop in voltage of the transmitted external signals, particularly a drop in the power supply voltages, causing the driver ICs to malfunction.
The drop in voltage level of the transmitted power supply voltages increases in the downstream transmission direction, resulting in a difference between the drive signals output in the upstream from the driver ICs to the liquid crystal display panel and that of the downstream, such that the gradation between the upstream and the downstream varies, even if the same gradation is intended for the display. Consequently, the increase in resistance of the signal wires causes the display quality of the liquid crystal display apparatus to deteriorate.
Although the placement and the shape of the signal wires formed on the TCPs in the crystal display apparatus described in Japanese Laid-Open Patent Publication No. 2001-56481 are devised in such manner as to decrease the resistance of the signal wires, such measures are not necessarily sufficient.